MDEV-19134: EXISTS() slower if ORDER BY is defined

Step 1: Removal of ORDER BY [LIMIT] from the subquery should be done
earlier and for broader class of subqueries.

The rewrite was done in Item_in_subselect::select_in_like_transformer(),
but this had problems:
- It didn't cover EXISTS subqueries
- It covered IN-subqueries, but was done after the semi-join transformation
  was considered inapplicable, because ORDER BY was present.

Remaining issue:
- EXISTS->IN transformation happens before
  check_and_do_in_subquery_rewrites() is called, so it is still prevented
  by the present ORDER BY.

1 files changed, 0 insertions, 15 deletions

diff --git a/sql/item_subselect.cc b/sql/item_subselect.cc
index e32a730214a..aaf2aa60fc6 100644
--- a/sql/item_subselect.cc
+++ b/sql/item_subselect.cc
@@ -3204,21 +3204,6 @@ Item_in_subselect::select_in_like_transformer(JOIN *join)
 
   DBUG_ENTER("Item_in_subselect::select_in_like_transformer");
   DBUG_ASSERT(thd == join->thd);
-
-  /*
-    IN/SOME/ALL/ANY subqueries aren't support LIMIT clause. Without it
-    ORDER BY clause becomes meaningless thus we drop it here.
-  */
-  for (SELECT_LEX *sl= current->master_unit()->first_select();
-       sl; sl= sl->next_select())
-  {
-    if (sl->join)
-    {
-      sl->join->order= 0;
-      sl->join->skip_sort_order= 1;
-    }
-  }
-
   thd->where= "IN/ALL/ANY subquery";
 
   /*